allow banners or campaigns to run custom code server-side to decide whether to fail. (aka cancel or hide)

maybe use lua?

useful for e.g. stewvote and other banners that have made API requests. see also T374394

https://meta.wikimedia.org/w/index.php?search=insource%3A%2F%5B%5EA-Za-z0-9%5D%28mw%5C.%28Foreign%29%3F%28Api%7CRest%29%7Capi%5C.php%29%5B%5EA-Za-z0-9%5D%2F+intitle%3Acentralnotice&title=Special:Search&profile=advanced&fulltext=1&ns8=1